The present invention relates to the art of heat treating and, in particular, to the controlled cooling of electrically conductive metals to achieve particular metallurgical characteristics.
The present invention has particular utility in the quench hardening of ferrous materials and will be described with reference thereto: however, it will become appreciated that the invention has broader aspects in ascertaining metallurgical cooling rates for other materials wherein the cooling rate affects the metallurgical characteristics of the heat treated parts.
Induction heating followed by liquid media quenching is a widely used technique for increasing the hardness of ferrous alloy parts. Such increased hardness may be provided as a surface treatment, for instance the journal area on a shaft, or to a substantial depth for parts experiencing high torsional, tensile and/or compressive loads. In all these cases, the requisite hardness is achieved by inductively heating the part to an elevated temperature above the critical temperature to provide an austenitic structure to at least the desired hardness depth. The heating is followed by a quenching period wherein the austenitic structure is transformed into a martensitic structure without formation of other transformation structures. In order to avoid the undesired transformation products, an adequate cooling rate is necessary, prescribed in a well known manner by the time-temperature-transformation (T-T-T) diagram for the particular alloys. Although critical to part acceptability, the cooling or quenching rate has not been a monitored in-line process parameter. Rather, adequate hardness has been determined through post-process destructive or non-destructive off-line testing and then for only a statistically selected number. Thus, the test does not provide current information for individually determining hardness, but rather provides an indication of the quality control for the tested sample lot. To increase the frequency of sampling has heretofore been deemed prohibitively expensive and time consuming.